As is well known, modern prospecting for hydrocarbons (i.e., oil and gas) commonly utilizes geological surveys to identify the location and depth of subsurface geological structures, based upon which the size and location of hydrocarbon reserves can be estimated. Common survey techniques involve the well-known seismic survey, in which acoustic energy is imparted into the earth and, after reflection or refraction by geological structures and their interfaces, is detected at multiple locations of the earth. The time delay between the impartation of the source energy and the detection of the reflected or refracted energy (i.e., the travel time) is used to estimate the depth of the geological structure from the surface. Of course, accurate conversion of travel time data into depth data requires accurate estimation of the acoustic velocity of the sub-surface strata.
As is well known in the art, so-called "process" velocities may be deduced from seismic survey data by way of certain well known techniques. Two such well known techniques are the constant velocity gather (CVG) and the constant velocity stack (CVS). The CVG and CVS processes each perform normal move-out ("NMO") for a set of 2-D seismic data over iterations of an assumed constant velocity; the closest stack velocity to the actual velocity is presumed to be that which results in the most realistic reflection artifact. Another well-known process velocity is the dip move-out ("DMO") velocity, which is used to correct for the angle of dip of a sub-surface reflector. Conventional migration techniques also produce a migration velocity, which is another type of process velocity, which is based upon the assumed location of equal velocity lines below the surface and above the reflector being migrated. According to each of these techniques, a seismic velocity survey may thus be readily generated from seismic data, providing excellent coverage of a relatively large survey region. The velocity survey is thus able to identify the depth of geological interfaces, based on the detected reflections of the acoustic energy and based upon the estimated acoustic velocity.
Each of these process velocities, as well as those obtained from core sampling and logging, are used to estimate interval velocities for a number of surface locations using the well-known Dix equation. While this approach results in a calculated interval velocity, both the process velocity values and the Dix equation itself are based upon assumptions about the sub-surface geology. These assumptions may not be valid for the actual survey volume, however, due to differences between the assumptions and the actual geology which manifest themselves as anomalies or artifacts in the interval velocity model for the survey. Examples of physical causes of these artifacts include those well-known problems due to near-surface layers, such as permafrost; near-surface layers of high velocity and significant thickness variation can especially causes problems in the stacking of seismic data.
Furthermore, considering that process velocity values are generally optimized to provide the best image of a reflector in the seismic data rather than the most accurate representation of the actual acoustic velocity in the earth, use of process velocities in performing time-to-depth conversion of seismic data is vulnerable to error.
Still furthermore, inaccuracy in the seismic velocity determination also results from the velocity anisotropy of the earth, such that the acoustic velocity in a direction normal to the surface of the earth differs from the acoustic velocity in a direction parallel to the surface of the earth. The CVG and CVS techniques are thus known to introduce inaccuracy in the determination of acoustic velocities.
As is also known in the art, the most accurate depth determinations may be made by way of well logging, in which direct measurements are taken from within a wellbore. The depths of interfaces between geological structures may be accurately determined from the well log, for example by measuring the depths at which discontinuities in the logged acoustic velocity occur. However, deviations are often present between the actual interface depths as indicated by well log measurements, on one hand, and the estimated depth of such interfaces deduced from a seismic velocity survey, on the other hand.
While well log measurements are of higher accuracy that depth profiles from seismic surveys, well log measurements are of course spatially limited to the location of the wellbore, and can provide no information at any significant distance away from existing wells. Therefore, much reliance is placed upon seismic surveys to identify productive locations at which new wells may be drilled; however, as noted above, deviations in the calculated and actual velocities and depths limit the accuracy of such seismic surveys.
It is therefore an object of the present invention to provide a method and apparatus for adjusting seismic velocity depth surveys to fit actual measurements taken at existing well locations.
It is another object of the present invention to provide such a method and apparatus which can perform such adjustments in an automated manner.
It is another object of the present invention to provide such a method and apparatus that can also determine the extent of error in the seismic survey following the adjustment.
Other objects and advantages of the present invention will be appreciated by those of ordinary skill in the art having reference to the following specification together with the drawings.